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Along similar lines, it is well established that there is a general tendency for CO₂ enrichment to change the composition of plants, and in particular to reduce nitrogen (and hence protein) concentrations in above‐ground plant tissues (Cotrufo et al., 1998), which may in turn have indirect effects on plant–animal interactions, because insect herbivores may then eat more foliage to maintain their nitrogen intake and fail to gain weight as fast (Fajer, 1989). CO₂ enhancement may also reduce concentrations in plants of other essential nutrients and micronutrients (see Section 3.5), contributing in turn to ‘micronutrient malnutrition’, which diminishes the health and economy of more than one‐half of the world’s human population (Loladze, 2002).

Examples of the effects on nitrogen content are shown in Figure 3.24. First, when wheat was grown at enhanced CO₂ concentrations, effects depended on whether soils had adequate nitrogen (low‐N) or this was supplemented with further fertilisation (high‐N) (Figure 3.24a). In the low‐N treatment, the protein concentration of individual grains was substantially reduced, but enhanced growth overall meant that the protein harvested from the grain was much less reduced. Indeed, in the high‐N treatment, the effect on concentration was negligible, such that the protein harvested overall from the grain was increased by CO₂ enhancement. On the other hand, it was possible to pinpoint the reason for the lowered nitrogen content under CO₂ enhancement, namely a reduction in nitrate assimilation efficiency (Figure 3.24b) – likely to be a very common cause of nitrogen depletion in tissues following CO₂ enhancement. Thus, while it may be possible to ameliorate the effects of future (and present) rises in CO₂ concentrations by increased nitrogen fertilisation, this reduction in assimilation efficiency will substantially increase the fertilisation costs and the polluting levels of unassimilated nitrates in soil run‐offs.

Figure 3.24 Effects of CO₂enhancement on plant protein concentrations with potentially harmful consequences. (a) The ratios of grain protein concentration (left) and the total protein‐nitrogen harvested from wheat, Triticum aestivum (right) in a free air CO₂ enrichment facilities (FACE) experiment comparing enhanced to ambient CO₂ concentrations at low and high soil nitrogen levels. Bars are SEs. (b) The proportion of nitrogen in wheat leaves that was nitrate over the course of this same experiment, which was consistently higher at elevated than ambient CO₂ concentrations (P < 0.0001), indicative of lower nitrate assimilation at elevated concentrations. Bars are SEs. (c) The protein content of goldenrod (Solidago spp.) pollen in relation to atmospheric CO₂ concentration from museum collections from 1842 to 1998 (left, r² = 0.81, P < 0.001) and from a FACE‐type experiment generating a CO₂ gradient (right, r² = 0.88, P < 0.01). Bars are SEs.

Source: (a) After Kimball et al. (2001). (b) After Bloom et al. (2014). (c) After Ziska et al. (2016).

A second example looked at the effect of increased CO₂ concentrations on the protein content of pollen from goldenrod plants, Solidago spp., in the USA, widely acknowledged by apiarists to be essential for the health and winter survival of both native bees (e.g. Bombus spp.) and honey bees (Apis melifera). Data both from historical records of pollen collected as CO₂ levels have risen, and from experiments that used a FACE‐like facility to establish a CO₂ gradient, showed that the protein content of the pollen was reduced substantially by increases in CO₂ concentrations (Figure 3.24c). These reductions could have serious effects on bee numbers, on pollination rates and hence on plant productivity, but the generality of these effects, the abilities of bees to mitigate them through changes in their own behaviour, and indeed the extent of the harm done to bees all remain to be determined. Examples such as these, therefore, emphasise both how profound the potential implications of CO₂ increases may be, in their own right, for future food security, and how difficult these implications can be to predict.